Equity Transactions - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended		6 Months Ended
	May 15, 2018	May 01, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity, Class of Treasury Stock [Line Items]
Common stock cash dividends declared per share (in $ per share)	$ 0.45		$ 0.45	$ 0.45	$ 0.90	$ 0.90
Preferred stock cash dividends declared per share (in $ per share)		$ 1.8125	$ 1.81	$ 1.81	$ 3.63	$ 3.63
Shares available for repurchase (in shares)			823,134		823,134
Shares authorized for repurchase (in shares)			4,180,045		4,180,045
Repurchases of common shares (in shares)					60,622
Weighted average price (in dollars per share)					$ 123.69
Total cost of shares repurchased					$ 7.5
Public offering 1 | Convertible preferred stock
Equity, Class of Treasury Stock [Line Items]
Convertible preferred stock dividend rate					7.25%